Medium voltage contactor versus Breaker 1

[kssschsekhar]

We are planning to install 500 Kw,11 kv MV motors in a power plant. For this best suitable power supply switching equipment is

1. MV Vacuum Breaker
or
2. MV Vacuum Contactor.

to start as DOL start.

I found that

Contactor is having following advantages comparing to Breaker

1. High number of electrical switching
2. Less installation cost.

Disadvantages
1. Less Breaking capacity (8000 Amperes)
2. Additional protective device like MV HRC fuse, load break switch require

Can anybody give furthur advantages and disadvantages of contactor comparing to breaker

Which is best equipment for long life and reliable purpose

 

[burnt2x]

On MV system plant equipment switching, VCB lords over other types, IMO. The VCB duty with lesser on/off cycles makes it desirable.
But one downside with using a very fast switch (VCB) is the concern regarding unusually high surge transients. If you go for fast switching VCB, be sure to specify surge arrestors to protect your load.

 

[slavag]

Hi Kssschsekhar .
Please find attached thread.
thread279-200366
For my pinion lot of usefull information.
Best Regards.
Slava

 

[ScottyUK]

thread238-199460 is worth a look too. 500kW at 11kV doesn't often make economic sense. You should be thinking in terms of 3.3kV or 4.16kV or whatever the local equivalent is for a motor of that size. 11kV switchgear is expensive and so are custom motors. Better to use a cheaper LV board and an off-the-shelf motor at the lower voltage.


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If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

ksekhar:

You already have summarized the most important pros and cons. The frequency of starts should decide which scheme to use as it will also directly affect your maintenance costs. Contactor/fuses are much cheaper and easier to maintain and replace if you are going to start stop the motor frequently. I am also assuming that you will have a motor protection relay in either case.

ScottyUK: If 11kV level already exist , they may be better off using it rather than introducing another level of transformation which has it own costs.

 

[slavag]

Hi.
For my pinion, today is cheaper order block trafo 11/3.3kV - motor. But of course,have check prices.
Regards.
Slava

 

[kssschsekhar]

rbulsara I agree with you. For selection start stops also matter.

Here I am looking advantages verses disadvantage.

For Technically speaking going for breaker is best with surge protection devices and you are know that contactor can be used upto 1000 kw motor only. Where is breaker is not having limitation for Motor starting duty.

I am looking for all technical reasons for selecting contactor or breaker.

Can any expert/manufacturer suggestion with information. It will be useful for all of us.

 

[ScottyUK]

Hi rbulsara,

I have serious doubts whether an 11kV starter lineup with custom motors would work out cheaper than an 11kV breaker with a 11/3.3kV transformer and a 3.3kV MCC, but a study of the economics would prove it one way or the other. Hopefully whoever does the study will look at lifetime costs rather than just initial build.

A couple more thoughts regarding the operation & maintenance aspects: PD activity exists at 11kV where it doesn't exist at 3.3kV, or not to any level worthy of comment. Jointing and terminations at 3.3kV are little more than oversized LV components, whereas 11kV joints and terminations are more complex and require skilled installation.


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If we learn from our mistakes I'm getting a great education!

 

[rcw retired EE]

Breakers have higher interrupting capability. Contactors use fuses to trip on short circuit.

For a 500 kw motor, you will have a small wire. The actual wire size may have to be larger with a circuit breaker protecting it. The breaker takes 3-5 cyles to clear a fault in the motor while the fused contactor does it in 1/2 cycle. Compare the cables short circuit withstand curves against the fuse and circuit brekaer clearing curves and you might see larger cables are needed with breakers to prevent shield or conductor damage during faults in the motor.

 

[rbulsara]

ScottyUK and all:
I was just pointing out one more consideration. All of your and others points are valid considerations. In the end an actual cost analysis need be performed to see where it stands. Quantity of motors would also affect the economy.

ksekhar:
I am not too sure breakers are always technically superior. As superiority is all relative to the application. For a 500kW motor breaker seems an overkill to tell you the truth. I am not proponent or opponent of any scheme, its all depends on what you are trying to accomplish.

 

[ScottyUK]

superiority is all relative to the application.

The best comment so far.


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If we learn from our mistakes I'm getting a great education!

 

[kssschsekhar]

Dear all

Using other than 11kv voltage in our system is ruled out. only 11kv switchgear acceptable in our case. (I request you don't think changing of voltage in this case only).

We found after the forum discuss additioanl information
With "contactor starting"
Advantages
a. Short circuit fault can be cleared fast. (less than 1 cycle). (we are known indirectly)
b.We can use less size cable

I will update if I found any other advatages.

Thank you for all for the information provided.